1. Field of the Invention
This invention relates in general to backlit displays, and more particularly to the illumination of a generally planar light transmissive image by light from a plurality of elongated tubular lamps in spaced parallel relation to each other and to said image. The displays of the invention incorporate modular sectional components and provide illumination of an image which is free of light patterns or shadows discernable to the human eye.
2. Description of the Prior Art
Large backlit displays currently used for illumination of translucent display panels bearing, for example, photographic scenes for indoor viewing in environments such as museums, airports, tradeshows and the like, suffer from a number of problems.
Current practice in such displays has been to use therein an array of equally spaced fluorescent lamps whose axes are parallel and are spaced from the plane of a translucent image bearing panel a distance generally equal to the distance between the lamp axes. This has lead to the use of displays having light box depths of about 10 to 12 inches. Such installations are massive in appearance, and are costly to manufacture, ship and install. Moreover, the currently available backlit displays supplied for the environments mentioned, provide inadequate illumination uniformity, the inferior quality of which is perceivable to the human eye, under certain conditions, as light patterns or shadows.
In my research aimed at producing ultimate perfection in uniform illumination of backlit displays, I found that the eye seems to see brightness nonuniformities to varying degrees depending upon the size of the image, the tonal density of the image, and the percentage of constant tonal density within a given image.
Prior art patents of which I am aware, which have addressed the problem of uniformity of backlit type illumination, have dealt with devices using one or two lamps in small backlit displays. One such patent states that a 10% range of variation in brightness is not detectable by the human eye. My experience, working with larger multi lamp displays, shows that a 10% brightness variation is objectionably visible in all types of large photographic scenes. The reason for this is apparently a psychological one, perhaps because large multi lamp displays show an extended repeat pattern of density variations which the eye can detect more readily than the one or two bright regions found in the one or two lamp small displays of the prior art.
In my copending U.S. Pat. No. 5,282,117. I disclosed various embodiments of a variable opacity masking system for use in a backlit display to block a first percentage of the light flux that is directed from a lamp therein along a plane which includes the lamp axis and the longitudinal axis of the mask and is normal to the display image panel. That masking system substantially reduces the intensity of the light at the portion of the image surface closest to the lamp, i.e., in the aforementioned plane. A maximum of light is blocked by the opacity of the mask at said plane, and the opacity of the mask is symmetrically reduced at increasing lateral distances on each side of its longitudinal axis, reaching zero at the margins of the mask.
The masking system of my earlier application aforementioned produced excellent results in light boxes of 5 inch depth in which the lamps were spaced apart 10 inches. Masks in planar form were positioned anywhere between the surface of the lamp and a distance of about 2 inches in front of the lamp axis, the space between the margins of the mask increasing with the distance of the mask from the lamp axis. Thus, the mask was wider at the 2 inch distance and narrower at the point of tangency with the lamp surface. Since the masks in planar configuration required separate planar light transmissive supporting means, a semicylindrical mask wrapped around and attached directly to the surface of the lamp became the preferred embodiment, because it involved fewer parts and lower manufacturing cost.
Both the planar and the semicylindrical shaped masks accomplished significant reductions in the bright pattern of lamps superimposed onto the image panel of prior backlit displays. For example, in the light box geometry mentioned above, the lamps created a brightness differential of about 15 percent when no masking was used. However, the same lamps, when provided with the masking means of my prior application, exhibited a dramatically reduced brightness differential of less than 3 percent. This represents a five-fold reduction in brightness differential and a substantial improvement over the 10 percent brightness differential stated as acceptable in the prior art.